Process for bulk sterilization, minimizing chemical and physical damage

ABSTRACT

Technique for bulk-sterilizing a wide range of substances by pulsed monochromatic electron beams of rather critical irradiation dose values and rates that enable effective destruction of viable micro-organisms and the like but without significant chemical or physical damage to the substance.

United States Patent Nablo Dec. 18, 1973 PROCESS FOR BULK STERILIZATION, MINIMIZING CHEMICAL AND PHYSICAL DAMAGE Inventor: Samuel V. Nablo, Lexington, Mass.

Energy Sciences, Inc., Burlington, Mass.

Filed: Oct. 4, 1971 Appl. No.: 186,427

Assignee:

US. Cl 21/54 R, 99/221, 162/192, 99/212 Int. Cl. A231 3/26, A611 1/00, A230 3/06 Field of Search 21/54 R; 99/221, 99/212; 250/495 TE References Cited UNITED STATES PATENTS 7/1952 Robinson 21/54 R X FOREIGN PATENTS OR APPLICATIONS 945,801 1/1964 Great Britain 21/54 R Primary Examiner-Joseph Scovronek Assistant Examiner-Tim Hagan Att0rneyRines and Rines 4 Claims, No Drawings PROCESS FOR BULK STERILIZATION, MINIMIZING CHEMICAL AND PHYSICAL DAMAGE The present invention relates to electron-beam bulk sterilization techniques, being more particularly concerned with pulsed electron beams. The invention teaches new methods of sterilizing and preserving substances whereby the destruction or inactivation of harmful or undesirable micro-organisms can be conducted without causing significant damage or other del eterious effects in the host or carrier medium, including bacteria, virus, yeasts, molds, enzymes and a number of other radiation-sensitive chemicals and protein-rich materials, including blood serum.

Previously, physical sterilization or other treatment of products has been effected by the use of radiation from high-energy machines or from radioisotope sources, such as cobalt 60, at reduced dose rate, as described, for example, in Proc. Symposium on Radiosterilization of Medical Products, Budapest, June 1967; IAEA Publication STl/PUB/l57, Vienna, Austria, 1967. For the case of radioisotope sources, because of the limited radiation intensity possible per unit volume of the source (specific activity), the dose rates available from large sources of industrial interest are typically in the range of 10 l rads per second. For the usual treatment levels of -10 rads for inactivation of microflora, to levels above rads used for sterilization, these rates indicate treatment periods of the order of hours which are presently used in such installations as, for example, in the terminal sterilization of some medical goods. Because of the inefficiences involved in machine-made X-radiation sources, these systems have not enjoyed wide commercial application for sterilization higher process efficiencies being possible using electron beams directly. Industrial electron beam accelerators (both of the dc. and pulsed linear type) can provide dose rates over limited fields of the order of 10 rads/second. In spite of their apparent improvement in process capacity, and the great commercial advantages of room temperature processing, however, such machinery has found limited application for industrial sterilization or similar processing due to the deleterious effects encountered in the processed material. Such effects have ranged from severe organoleptic changes in foodstuffs, denaturation of proteins, chemical alteration of carbohydrates, hydrolysis of aqueous solutions, discoloration of cellulose and other natural products, and other disadvantageous effects.

Product sterilization by such prior techniques has often produced deleterious physical changes in the product such as cross-linking of plastic films, degradation of adhesive coating and discoloration.

In U. 5. Pat. Nos. 2,429,217; 2,807, 551; 2,456,909; 2,617,953 and 2,796,545, treatment techniques are described involving high-speed wide-spectrum electrons with irradiation periods less than 10 seconds. These equipments enable the production of intense beams of unknown quality (energy distribution), generally over large areas and at energies of up to 6 million volts. Practical demonstrations with minimal analytical support were described and it was concluded that the bactericidal and material properties of the process were little affected by repetitive treatment so long as the individual period of exposure was held short. This pioneering work, however, possibly because, among other reasons, of the inherent concurrent damage to the chemical and physical properties of the treated substances, did not find commercial usage.

In accordance with the present invention, however, it has been discovered that it is possible effectively to destroy viable micro-organisms and pathogens and the like in a wide variety of substances without damage to the physical and chemical structure, and properties of such substances by using pulsed, controlled-spectrum electron beams with rather critically adjusted irradiation doses and rates.

An object of the invention, accordingly, is to provide a new and improved process of electron-beam sterilization or irradiation, not subject to the disadvantages, before discussed, but effective without damage to the irradiated substance structure.

A further object is to provide a novel electron-beam technique of more general applicability, as well.

Other and further objects will be explained hereinafter and are delineated more fully in the appended claims. In summary, from one of its broad aspects, the invention contemplates a process for bulk sterilization involving selectively destroying viable micro-organisms and the like in a medium, such as cellulose-containing products, blood, pharmaceuticals, cosmetics, foodstuffs and other substances, but without effecting substantial chemical and physical damage to the media, that comprises, disposing a medium to-be-sterilized at a predetermined region, generating pulsed substantially monochromatic electron beam energy and directing the same to said region substantially uniformly to irradiate the medium, and adjusting the pulses of energy to produce an irradiation dose of value within the range of from substantially a few tenths to several megarads and with a dose rate greater than substantially l0 rads per second. Preferred details are hereinafter set forth.

In accordance with the invention, apparatus of the types described in my copending United States applications, Ser. Nos. 31,530, filed Apr. 24, 1970, and now US. Pat. No. 3,612,941 issued Oct. 12, 1971, for Pulsed Field Emission Cold Cathode With Means For Replacing Stripped Adsorbed Gas Layer, and 64,734, filed Aug. 18, 1970, now US Pat. No. 3,720,838, issued Mar. 13, 1973, for Apparatus For and Method of Controlling Relativistic Charged Particle Beam Distribution and Transport and in Observations of Magnetically Self-Focusing Electron Streams," S. V. Nablo, Appl. Phys. Lett. 8, No. 1, 18 (1966), and The Generation and Diagnosis of Pulsed Relativistic Electron Beams Above 10 Watts, S. E. Graybill and S. V. Nablo, IEEE Trans. Nuc. Sci. NS-l4, No. 3,782, (1967), may be used with rather critically adjusted and controlled small-area pulsed electron beams of high current density (as distinguished from high current) and consequently of very high dose rates, to effect these novel results. In view of the known and published characteristics of such controlled electron-beam generators, it is not deemed necessary to illustrate the same in this application, the invention being concerned rather with the novel and unexpected application of this type of equipment, operated within certain novel adjustments, to attain such results.

While, as later explained, dose rates considerably in excess of 10 rads per second are required for the purposes of the invention, with irradiation doses of from a few tenths to several megarads for substantially monochromatic, short-period, high intensity beams, producing single-pulse doses for a 50 nanosecond system, for

example, typical dose rates here-employed are of the order of 4 X 10" rads/second. Rigorous demonstration of the retention of micro-organism destructive efficacy at dose rates up to 10 rads/sccond, however, was achieved, for example, on anaerobic spore formers, aerobic spore formers, vegetative bacteria, yeast, virus, enzymes, molds and the like. Comparative studies for vegetative bacteria and spore formers, (bacillus pumilus and subtilis, micrococcus radiodurans, and streptococcus faecium) with conventional treatment technique at 10 l rads/second (before-mentioned) and with the present invention (at rads/second) demonstrated equivalent killing results and demonstrated that the resistance of these micro-organisms is quite insensitive to the rate of treatment, their survival being determined, rather, by the integrated dose or energy per unit mass which they receive. In the case of some process applications, enhanced effects were seen with the pulsed high rates of the invention, as, for example, in enzyme inactivation. Inactivation of the enzyme phosphatase in whole milk by the invention, again compared favorably with the conventional radiation treatment techniques. In this instance, a very large reduction in organoleptic and color changes were observed, as the whole milk, known to be the most radiationsensitivc of foodstuffs, showed no detectable alteration in these properties at pulsed monochromatic beam sterilization treatment levels (2 megarads at 10' rads/second). Similar comparative results were obtained in thus inactivating the enzymes catalase L and lysozyme.

These unexpected results on the response of an animal protein-rich system led to studies on other radiation-sensitive structures, such as the quaternary ammonium surface active germicides, an example of which is alkydimethylbenzylammonium chloride. Comparative studies of radiation induced damage in this compound at high and low dose rates were conducted. Electron irradiations were performed with a 2.2 MeV beam, a fixed pulse duration of 30 nanoseconds and integrated single-pulse doses of 760 and 1,500 kilorads on 5 ml aliquots at room temperature. The studies were performed using apparatus similar to that before described with dose rates in the 10 rads/second range. Total sample thicknesses under I gm/cm were employed permitting adequately uniform irradiation for this application; i.e., front-to-back surface doses of unity using unidirectional irradiation. The low level Co irradiations were performed at a fixed dose rate of 100 rads/second, again at room temperatures and to integrated levels of 800 and 3,000 kilorads. These levels were chosen since lethality studies have shown the 1,500 kilorad figure to be adequate for the sterilization application with the pulsed electron beam although tests have also been conducted at higher levels. Ultraviolet spectroscopic evaluation of the material was used to determine the degree of damage to the active benzyl ammonium group; and the structures in the 2,000-2,200 A and in the 2,500-2,800 A regions characteristic of the benzylammonium radical demonstrated severe damage under low rate (C0 irradiation and negligible change under the high rate (electron) treatment of the invention.

Equally conclusive results were obtained for three megarad treatment levels. No change in the effective zone of inhibition was observed (standard ATCC method) for the electron-processed and for control samples (13.5 mm), while a severe degradation of the bacteriostatic activity of the cobalt-processed sample (7 mm) was observed.

The decreased chemical damage associated with the high rate electron beam treatment of the invention bears out the results of other experiments, moreover, particularly with proteins. It appears that the rapid quenching of the free radicals formed during the radiation pulse or bursts, ameliorates (in fact may eliminate) secondary damage effects in the system; while those primary damage mechanisms which dominate the bactericidal action are unaltered. The extensive lethality studies described with vegetative bacteria, aerobic spore formers, enzymes and virus have confirmed this latter effect; i.e., that the dose required to reduce the micro-organism population by a factor of ten varies little with rate in some cases it has been found to decrease. The radiation sensitivities of bacteria and microflora are known to be very dependent upon environment, notably oxygen availability. It is for this reason that parallel comparative studies were performed.

Based on these results, the effect of the techniques of the invention on the various fractions of human bloodplasma proteins was investigated. Plasma proteins and their fractions have been used for diagnostic and therapeutic purposes for many years. Since they are usually used parenterally, a sterile preparation is required. Conventional methods for protein sterilization are not completely satisfactory due to loss of proteins and their relative ineffectiveness in preventing virus contamination. The application of the present electron sterilization for cold treatment of proteins can eliminate losses currently involved in the filtration techniques commonly used. Results from comparative experiments on human fibrinogen and human plasma showed that significant reduction in the destruction of clottable protein in these blood fractions is attainable by the invention (l0 rads/second), as compared with the rads/second treatment level for Co.

Similar studies have been conducted on complex molecular structures of medical and commercial interest. Insulin has been given like treatment with similar results and is considered as a representative proteinbased injectable. Similar results for comparative treatment studies in commercially available pork insulin were obtained, using the iodine l25-labelled radioimmunochemical assay technique in determining the degree of irradiation-induced inactivation. Procaine hydrochlon'de has also been studied at sterilization levels with the technique of the invention and has been found to suffer no deterioration. This material represents an important base for a number of injectables in widespread daily use. Similar injectables which cannot be heat-sterilized because of their susceptibility to hydrolysis (such as thiopentol sodium, pentabarbitol sodium, penicillin, etc.) and which must be refrigerated in order to extend their shelf life, can also be successfully treated by the high dose rate technique taught herein.

In summary then, the invention has provided a process whereby materials sensitive to heat or hydrolysis can readily be sterilized without physio-chemical damage to the treated compound.

While the process has been described in application to sterilization and preservation of various substances, including foodstuffs and pharmaceuticals, it is not limited to these areas, alone, since the single-pulse high dose rate processing of materials otherwise sensitive to physio-chemical damage offers a practical technique to their sterilization or pasteurization (radappertization or radurization In the latter application, for example, the decreased sensitivity of animal protein to the process will permit the increased storage time of meats to several months without freezing; e.g., with storage at 3C and a treatment level of 0.5 megarads. The radappertization of whole milk, cultured dairy products and other foods ispossible with the enhanced enzyme inactivation (and equivalent lethality to bacteria and microflora) demonstrated with phosphatase and catalase. In its application, moreover, one need not resort to the use of sensitizers to decrease the effective dose (because of organoleptic effects); such sensitizers have involved freezing, chemicals, antibiotics, chemical preservatives, etc.

Further modifications will occur to those skilled in this art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

I claim:

I. A process for bulk sterilization involving selectively destroying viable micro-organisms and the like in a medium but without effecting substantial chemical and physical damage to the medium, that comprises, disposing a medium-to-be-sterilized at a predetermined region, said medium being selected of sample thickness of the order of approximately one gram per square centimeter and under, generating pulsed substantially monochromatic electron beams and directing the same to said region substantially uniformly to irradiate the medium, and adjusting the pulses of electron beam energy to produce an irradiation dose of value within the range of from substantially a few tenths to several megarads and with a dose rate greater than substantially l0 rads per second.

2. A process as claimed in claim 1 and in which the electron beam energy is adjusted to a value in excess of substantially 1 MeV.

3. A process as claimed in claim 1 and in which the dose range is adjusted between substantially l and 4.5 megarads.

4. A process as claimed in claim 1 and in which the said medium comprises at least one of blood, pharmaceuticals, cosmetics, foodstuffs and cellulosecontaining products. 

2. A process as claimed in claim 1 and in which the electron beam energy is adjusted to a value in excess of substantially 1 MeV.
 3. A process as claimed in claim 1 and in which the dose range is adjusted between substantially 1 and 4.5 megarads.
 4. A process as claimed in claim 1 and in which the said medium comprises at least one of blood, pharmaceuticals, cosmetics, foodstuffs and cellulose-containing products. 